1. Field of the Invention
The present invention relates to electrothermal printing apparatus in which printing is effected by momentarily heating selected portions of a heat sensitive medium, and more particularly to arrangements in which characters and other indicia are printed on thermally sensitive paper by imparting heat to the paper via an array of heads or other energizable elements movable relative to the paper.
2. History of the Prior Art
Electrothermal printing apparatus in which one or more heads or other elements are momentarily heated to heat selected areas of an adjacent thermally sensitive paper or other thermally sensitive medium which discolors in response to the heat to effect printing is well known in the art. In typical arrangments of this type a row of side-by-side heads is often provided for sweeping movement relative to the thermally sensitive paper to effect printing of characters or other indicia in dot matrix fashion. The individual heads typically consist of small resistive elements which must be heated to a temperature high enough to color the paper to the desired degree of resolution. At the same time heating of the head must be done relatively quickly so that only a discrete localized area of the paper is colored as the paper continues to move relative to the heads. Examples of this type of printing apparatus are provided by U.S. Pat. No. 3,951,247 to Montanari, ELECTROTHERMAL PRINTING UNIT, issued Apr. 20, 1976, U.S. Pat. No. 3,989,131 to Knirsch et al, ELECTROTHERMAL PRINTING UNIT, issued Nov. 2, 1976, and U.S. Pat. No. 3,967,092 of Conta et al, ELECTROTHERMAL PRINT HEAD, issued June 29, 1976.
Conventional electrothermal printing units have been found to involve a number of problems in their design and operation. One such problem stems from the fact that the growing need for greater resolution requires smaller heads which can be heated to higher temperatures over shorter periods of time. The rapid heating of the relatively small heads to relatively high temperatures produces the requisite resolution and printing speed but at the expense of greatly shortened head life as the resistive heating elements within the heads deteriorate quickly. A further problem which greatly shortens head life results from the fact that the heads must usually be maintained in physical contact with the thermally sensitive paper to provide the desired resolution. The surface of such paper tends to be rather abrasive, resulting in premature head wear.
Problems of this type have led to consideration of alternative approaches such as where the electrically resistive heating elements are combined into a single ribbon or like member heated at selected areas by an arrangement of energizable electrodes. Such arrangements typically utilize a meltable or otherwise thermally sensitive layer of ink or other coloring material imposed on a surface of the resistive ribbon so as to melt and impart color to a contacting piece of paper. Printing arrangements of this type avoid some of the severe head wear problems present in other types of systems but at the expense of certain problems of their own, not the least of which is the rather poor resolution that often results from the extreme difficulty in heating a small and well defined portion of the ink to a selected degree. Aside from the rather complex ribbon configuration which results from the presence of an ink layer in addition to electrically resistive and conductive layers, such arrangements are frequently incapable of localizing the heating to a small discrete area of the ribbon or of heating different areas of the ribbon uniformly. An example of an arrangement which attempts to solve this type of problem by providing each head with a pair of uniform, closely spaced electrodes is provided by U.S. Pat. No. 3,744,611 of Montanari et al, ELECTRO-THERMIC PRINTING DEVICE, issued July 10, 1973.
Other examples of printing apparatus, some of which attempt to heat a resistive element unattached to the head electrodes, are provided by U.S. Pat. Nos. 3,848,720 of Carlsen, PRESSURE SPRING FOR A THERMOPRINTER, issued Nov. 19, 1974, 3,984,844, of Tanno et al, THERMAL RECORDING APPARATUS, issued Oct. 5, 1976, 4,056,822 of Thornburg et al, LOW PROFILE SINGLE CHANNEL THERMAL ANALOG RECORDER, issued Nov. 1, 1977, 4,030,408 to Miwa, THERMAL PRINTER HEAD, issued June 21, 1977, 3,719,261 to Heinzer et al, PRINTING METHOD AND APPARATUS USING CONDUCTIVE FUSIBLE INK, issued Mar. 6, 1973, and an article by J. L. Mitchell and K. S. Pennington, ELECTRICAL AND MECHANICAL MECHANISM FOR THERMAL TRANSFER PRINTING, IBM Technical Disclosure Bulletin, Vol. 18, No. 8, January 1976, pp. 2693-4.
Despite the localized and relatively uniform heating provided by the arrangements shown in Montanari et al, arrangements of this type still lack the resolution required, particularly in relatively high speed applications. Further problems arise from the fact that the ink coated ribbon often cannot be reused with good results because of the destructive nature of the process in which the ink is melted and flowed onto the adjacent paper.